Resonators are suitable for use in bandpass filters in modern filter technology and may, for example, be used in mobile communication devices.
A resonator working with bulk acoustic waves has a piezoelectric layer arranged between two metal layers (electrodes). The layers are sequentially deposited on a substrate and structured in such a manner that there arises a plurality of resonators, which are electrically connected by their correspondingly structured electrodes and may together realize a filter circuit, for example.
To store the acoustic energy of the bulk acoustic wave within the resonator and/or to keep acoustic energy in the substrate upon which the resonator is arranged from escaping, either an acoustic mirror is arranged underneath the resonator or an air gap is provided. An acoustic mirror includes at least two, but maybe more layers, alternately comprised of materials having high and low acoustic impedance. The material and the thickness of these layers are so chosen that, for the selected resonant frequency of the resonator, all layer thicknesses are in the range of a quarter wavelength (or an odd multiple of a quarter wavelength) of the acoustic wave that can propagate in the respective material at this frequency. Under these conditions, the theoretical ideal case of maximum constructive interference of the acoustic waves reflected on the boundary layers is approximately optimized and thus prevents the escape of acoustic energy from the resonator.
An air gap also serves the same purpose, because the large impedance jump between the top (or bottom) layer of the resonator and the air is sufficient to reflect the acoustic wave almost completely.
A resonator working with bulk acoustic waves or a component that exhibits such resonators is just as sensitive as a surface acoustic wave (SAW) component to mass load or to damage from contaminating substances on the surface of the resonators. Component housings known from semiconductor technology in which the components are built-in, glued-in, for example, are thus normally used for such FBAR resonators and components. Such housings, which include ceramics or metal, for example, have two parts, comprised, for example, of a trough and a lid or a carrier and a cap, the two parts of the housing being glued, welded, or soldered together after the component has been inserted.
Such housing technologies demand high procedural and financial costs, however, and cannot keep up with the miniaturization of the components because of the required minimum dimensions, of wall thicknesses, for example, this miniaturization being required for technological and economic reasons. It is therefore proposed in U.S. Pat. No. 6,087,198 that the packaging be replaced by an external seal using a plastic material. To prevent the plastic seal from having a negative effect on the acoustic properties of the resonator, an acoustic mirror is provided between the resonator and the seal. U.S. Pat. No. 5,872,493 also describes a seal, which is deposited over the component and comprises at least one passivation layer comprised of SiO2, an epoxy resin or a desired glob-top composition. Here too, an acoustic mirror comprising at least three mirror layers is inserted therebetween to prevent acoustic interference with the resonator.